JPH021894B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a method for manufacturing a high-strength cold-rolled steel sheet having a strength of over 100 Kgf/mm ² and excellent cold workability and weldability. <Prior Art> In recent years, the use of high-tensile steel plates has been increasing in the automobile industry due to the need to reduce the weight of vehicle bodies to improve fuel efficiency and to ensure passenger safety in the event of a collision. In particular, from the point of view of safety in the event of a collision, a steel plate with a much higher tensile strength than conventional high-strength cold-rolled steel plates, which has a tensile strength of over 100 Kgf/mm ² , is required. Automotive steel sheets do not simply have high strength;
Cold workability and weldability are naturally required due to the intended use, and excellent cold workability and weldability are required even for steel plates with a tensile strength of over 100 Kgf/mm ² . In the production of high-strength cold-rolled steel sheets with a tensile strength of over 100 Kgf/ ^mm2 , conventional methods have been used such as water cooling or, if the cooling rate is slow, such as box annealing, increasing the amount of alloy added to increase the strength. It is being In the water cooling method, the cooling rate is too fast, so a large amount of strain accumulates in the steel plate during the cooling process, making it impossible to obtain excellent cold workability. However, in order to obtain a tensile strength exceeding 100 Kgf/mm ² , it is necessary to add a large amount of alloying elements, which has the disadvantage of impairing weldability. Steel sheets for automobiles require both cold workability and weldability, and conventional methods have been unable to produce steel sheets of 100kg that satisfy both of these requirements.
It has not been possible to produce a high-strength steel plate with a tensile strength of more than f/mm ² . <Object of the invention> The present invention improves the above-mentioned drawbacks and has ^excellent cold workability and weldability.
The present invention provides a method for producing a cold-rolled high-strength steel sheet having a tensile strength of f/mm ² or less. <Structure and operation of the invention> The present inventors have developed an invention that has both cold workability and weldability.
As a result of intensive studies on the manufacturing method of cold-rolled high-strength steel sheets with a tensile strength of over 100 Kgf/ ^mm2 , we determined that the cooling rate in the annealing process should be over 100°C/sec and below 500°C/sec, and the Mn content should be 1.4 to 500°C/sec. By adding a small amount of B and one or two of Ti and Nb in the range of 2.6%, the effect of B on inhibiting transformation can be increased as described above.
This is extremely noticeable in the range of Mn content, and the weldability is improved by the addition of Ti and Nb, and furthermore, in the range of small Mn content as mentioned above, weldability is improved to 100Kgf/mm ²
Because it has extremely high tensile strength, it has excellent weldability compared to its strength, and because the amount of strain that accumulates in the steel plate during cooling is small, it has an excellent shape and excellent cold workability. It was discovered that a steel plate with a certain temperature can be obtained. The present invention was made based on this knowledge, and its gist is that C: 0.10-0.20%,
Si: 0.6% or less, Mn: 1.4-2.6%, Ti: 0.01-0.25
%, Nb: 0.01 to 0.30%, and B: 0.0003 to 0.0100%, with the remainder being Fe and unavoidable impurities, after hot rolling and cold rolling according to a conventional method. A ₁ in the sintering process
1 second to 5 seconds in the temperature range above the transformation point and below 900℃
After holding for minutes, the average cooling rate exceeds 100℃/sec500
100Kg characterized by cooling below ℃/sec
The present invention provides a method for producing a cold-rolled high-strength steel plate having a tensile strength of more than f/mm ² and less than 150 kgf/mm ² and excellent cold workability and weldability. Figure 1 shows C: 0.12%, Si: 0.40%, Ti: 0.05%,
B: The basic composition is steel containing 0.0015% and no B, and the steel with the Mn content varied in the range of 1.2 to 3.0% is melted and hot-rolled and cold-rolled according to the conventional method to obtain a plate with a thickness of 1.0. mm steel plate and then at a temperature of 800℃
Hold for 40 seconds, then cool at 7℃/sec to 700℃, then cool at 200℃/sec.
FIG. 3 is a diagram examining the relationship between Mn content and tensile strength. From the figure, the tensile strength of the steel of the present invention to which B is added is 10 to 20 Kgf/mm ² in the Mn content range of 1.4 to 2.6% compared to the comparative steel to which B is not added.
Increase Mn to also obtain the same tensile strength
It can be seen that the amount can be reduced by about 0.4 to 0.6%. It can be seen that by adding B in this way, high tensile strength can be obtained with a small amount of Mn, and thus a steel plate with superior weldability compared to tensile strength can be manufactured. FIG. 2 is a graph showing the relationship between tensile strength and total elongation between the method of the present invention and the comparative method for the steel shown in FIG. 1 and some other steels. As can be seen from FIG. 2, the method of the present invention has a superior strength-ductility balance compared to the comparative method. This is because, although the tensile strength increases by adding B, there is little decrease in ductility. As described above, it can be seen that according to the method of the present invention, a high-strength cold-rolled steel sheet having a strength of more than 100 Kgf/mm ² which has a low alloy but has superior ductility compared to tensile strength can be easily produced. The reasons for limiting the components as described above in the present invention are as follows. C is an element necessary to obtain strength using precipitation strengthening and transformation strengthening, and its content is 0.10%.
If it is less than 0.1%, precipitation strengthening and transformation strengthening cannot be fully utilized and a tensile strength exceeding 100 Kgf/mm ² cannot be obtained, so the lower limit is set at 0.10%. Furthermore, if the content exceeds 0.2%, weldability will be significantly reduced, so the upper limit is set at 0.2%. If the Si content exceeds 0.6%, scale formation will occur significantly during the hot rolling process, deteriorating the surface properties of the steel sheet and deteriorating the chemical conversion treatment method, which is a necessary characteristic for automotive steel sheets, so the upper limit should be set. It was set at 0.6%. Mn is an important element for obtaining strength using transformation strengthening, and if its content is less than 1.4%, the amount of martensite produced is small and the desired tensile strength cannot be obtained, so the lower limit is set at 1.4%. do. Also,
If the content exceeds 2.6%, not only will weldability be significantly impaired, but the strength-increasing effect of B addition, which is a feature of the present invention, will be reduced, so the upper limit is set at 2.6%. Ti and Nb are not only elements necessary to obtain strength by utilizing precipitation strength, but also essential elements to effectively utilize a small amount of B, which is a feature of the present invention, and further improve weldability. It is also an essential element for improving B is an element that easily combines with N, and if there is solid solution N in steel, B becomes BN.
Therefore, precipitation B cannot be used effectively. On the other hand, since Ti and Nb have the ability to fix N as nitride, effective use of B becomes possible. Also,
Ti and Nb have the effect of refining the crystal grains in the heat affected zone during welding and suppressing the occurrence of cracks from the heat affected zone. Therefore, the steel of the present invention contains Ti or
Nb is an essential element. The content of Ti is
If it is less than 0.01%, the above-mentioned N fixation and weldability improvement are insufficient, so the lower limit is set at 0.01%, and
Even if the content exceeds 0.25%, the effect will be saturated, so the upper limit is set at 0.25%. Similar to Ti, if the Nb content is less than 0.01%, fixation of N and improvement of weldability are insufficient, so the lower limit is set at 0.01%, and if the content exceeds 0.30%, the effect will be saturated. Therefore, the upper limit is set at 0.30%. B is an essential element to take advantage of the characteristics of the steel of the present invention, and if its content is less than 0.0003%, it will not be effective in suppressing transformation and preventing a decrease in tensile strength, so the lower limit is set at 0.0003%. shall be. Also
If the content exceeds 0.0100%, flaws are likely to occur during the hot rolling process and the surface quality of the steel sheet will be significantly impaired, so the upper limit is set at 0.0100%. If sol.Al is less than 0.005%, pinholes will easily appear during casting and the surface quality of the steel plate will be impaired.
The lower limit is 0.005%, and if it exceeds 0.10%, surface defects due to alumina will increase and the surface condition of the steel plate will also be impaired, so the upper limit is 0.10%. The lower limit of the annealing temperature range is set to the _A1 transformation point because transformation strengthening cannot be utilized below the _A1 transformation point, and furthermore, the effect of a small amount of B, which is a feature of the present invention, cannot be utilized. The _A1 transformation point is approximately 690-730°C in the composition range of the steel of the present invention. Furthermore, since it becomes difficult to pass the plate in the continuous annealing process at temperatures exceeding 900°C, the upper limit is set at 900°C. The cooling rate is
Below 100°C/sec, the effect of B cannot be used effectively, and in order to obtain the desired tensile strength, the amount of alloy added must be increased, resulting in a decrease in weldability. do. Moreover, if it exceeds 500°C/sec, the amount of strain accumulated on the steel plate during cooling increases and it is difficult to obtain good cold workability, so the upper limit is set to 500°C/sec. If it is difficult to cool directly from the holding temperature within the above cooling rate range due to continuous annealing equipment, after gradual cooling, the temperature will be lower than 650℃.
The temperature range of 400°C or higher, preferably 300°C or higher, must be cooled at a cooling temperature of more than 100°C/sec and 500°C/sec or less. More preferably after cooling
Cold workability is improved by holding the material in a temperature range of 100 to 400°C for 10 seconds to 10 minutes. (Example) Next, an example of the present invention will be described. Steel having the chemical composition shown in Table 1 is melted in a converter, hot rolled at a finishing temperature of 860 to 900°C to a thickness of 2.5 mm, pickled and then cold rolled. A steel plate with a thickness of 1.0 mm was used. Then, the steel plates were annealed under various conditions, and the tensile strength and spot weldability of the steel plates obtained after annealing were investigated. Table 1 also lists annealing conditions. Table 2 shows the manufacturing conditions, mechanical properties, and fracture morphology after spot welding. As can be seen from Table 2, the desired tensile strength could not be obtained in Comparative Methods A and C because the components were outside the method of the present invention, and in Comparative Method B, although the tensile strength was obtained, the components were in accordance with the method of the present invention. In D and E, the desired tensile strength was not obtained because the cooling conditions were different from the method of the present invention. On the other hand, F and G, which are the methods of the present invention, have excellent strength-
It can be seen that it has a good ductility balance and has an excellent fracture morphology during spot welding. <Effects of the Invention> As described above, according to the present invention, it is possible to easily produce a high-strength cold-rolled steel sheet with good cold workability and weldability and a tensile strength of more than 100 Kgf/mm ² and less than 150 Kgf/mm ² can be manufactured.

【table】

【table】

【table】 [Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between Mn content and tensile strength when continuous annealing is performed for steel with and without B added, and Figure 2 is a diagram showing the relationship between Mn content and tensile strength produced by the present invention method and the comparative method. FIG. 3 is a diagram showing the relationship between tensile strength and total elongation of steel.

Claims (1)

[Claims] 1 C: 0.10-0.20%, Si: 0.6% or less, Mn: 1.4
~2.6%, sol.Al: 0.005~0.10%, Ti: 0.01~0.25
%, Nb: 0.01 to 0.30% or both, and B: 0.0003 to 0.0100%, and the balance is Fe and unavoidable impurities. After hot rolling and cold rolling according to a conventional method, the steel is subjected to an annealing process. After holding the temperature in the temperature range of _A1 above the transformation point and below 900°C for 1 second to 5 minutes, the average cooling rate exceeds 100°C/sec and 500°C/
100Kgf/mm ² characterized by cooling below sec
A method for producing a high tensile strength cold rolled steel sheet having a tensile strength of less than 150 Kgf/mm ² and excellent cold workability and weldability.